1. Field of the Invention
The present invention relates to a dot printer for printing characters, halftone images and the like with small pixels (dots).
2. Description of the Prior Art
Various recording element sections or printer heads for use in dot printers that form output images with dots are known. Examples of such heads include a wire dot printer head, an electrostatic printer head, an ink-jet printer head, a thermal printer head, an LED (light emitting diode) array printer head and the like. An LED array printer head having 8 to several tens of elements per mm as the dot generating elements is receiving the most attention due to its extremely high resolution. When this head is used in place of an optical scanning mechanism in a conventional electrophotographic copying machine, a printer can be realized in which an array of LEDs is selectively turned on in accordance with a video signal to form an electrostatic latent image on a surface of an adjacent photosensitive body and a visualized image is obtained through a transfer process of the image onto a transfer sheet. In a printer of this type, a portion corresponding to the ON LEDs can be formed into a black or white image portion by changing the charging conditions or toner.
FIG. 1 shows a block diagram of a conventional LED array printer head driver. The driver performs main scanning for electrically scanning the array of LEDs and subscanning for moving a photosensitive surface in a direction perpendicular to the main scanning direction so as to print out an image. Referring to FIG. 1, a data enable signal (DATA-EN) 7 enables main scanning and defines an interval in which a video signal (VIDEO) 5 is effectively printed out. More specifically, when the data enable signal 7 is at level "1", a counter 1 and a decoder 4 are enabled. The counter 1 starts counting main scanning clock signals (CLK) 2 and produces a count output signal 3. The decoder 4 decodes the count output signal 3 from the counter 1 and produces latch pulse signals 4.sub.1 to 4.sub.n in the order of values 0 to (n-1) of the count output signal 3. The video signal 5 is commonly supplied to data input terminals D of latch flip-flops (to be referred to as latches hereinafter) FF.sub.1 to FF.sub.n. The latch pulse signals from the decoder 4 are sampled in a predetermined order and are stored in the latches FF.sub.1 to FF.sub.n. N light-emitting diodes (to be referred to as LEDs hereinafter) LED.sub.1 to LED.sub.n are arranged adjacent to each other to constitute an LED array printer head 6. Drivers D.sub.1 to D.sub.n for individually driving the LEDs LED.sub.1 to LED.sub.n control the ON/OFF timing of the currents flowing thereto. For example, when the latch FF.sub.1 has stored a dot pixel signal of level "1", the driver D.sub.1 causes a current to flow to the LED.sub.1 and so turn it on. On the other hand, when the latch FF.sub.1 has stored a dot pixel signal of level "0", the driver D.sub.1 blocks current supply thereto to turn it off. Resistors R.sub.1 to R.sub.n control the currents flowing to the LED.sub.1 to LED.sub.n. In a head of this configuration, when printing in the main scanning direction is performed, the photosensitive surface is also scanned in the subscanning direction. In this case, the latches FF.sub.1 to FF.sub.n hold the stored pixel signals up to the next main scanning latch timing. Therefore, the signal holding time is the same in all LEDs. When subscanning is performed for a predetermined length of time during such signal holding time, an electrostatic latent image of one line is formed on the photosensitive surface. When the image is developed and is transferred onto a recording paper sheet, a visible image is formed which consists of black portions exposed to light from the LEDs and white portion not exposed to such light.
However, even if the same current is made to flow to the LEDs, there are variations in the light intensities of the respective LEDs. Such variations of LEDs are unavoidable at the present moment. Therefore, if the LEDs are driven in the same manner as in conventional techniques, the printed image has density irregularities due to such variations in the light intensity or luminance of the LEDs. This phenomenon occurs in various types of printer head including an LED array printer head. For example, in wire dot printers, which have recently become popular, a similar irregularity in the printed image occurs due to variations in the wear of the pin head mechanism and the electrical characteristics of the actuator. In a wire dot printer, the dot pin number is small, and the output image need not satisfy too high a resolution requirement. For this reason, the problem of irregular density of the printed image has been frequently solved by adjustment by a trained operator. However, in the case of applications which require a great number of output dots and a high resolution of the printed image, this problem cannot be easily solved.
A dot output device expresses an image by a combination of a number of output dot pixels. This means that a high-resolution image requires a great number of dot output elements. Furthermore, in order to provide gradation or gray levels with a dot output device, one pixel conventionally consists of a plurality of dot pixel outputs. The black dot output density within the pixel is variably controlled to provide a pseudo-halftone effect. For this reason, an image obtained by this method cannot have a high density and has not been satisfactory.